Passengers using elevators can give calls to elevators in an elevator car and/or in an elevator lobby. Elevator lobbies are typically provided with up/down pushbuttons, by means of which a passenger can order an elevator to the call floor and simultaneously indicate his/her travel direction. After the elevator has arrived at the call-giving floor, the passenger moves into the elevator car and indicates his/her destination floor with the pushbuttons of the car panel in the elevator car. To a constantly increasing extent so-called destination call systems are used in high-rise buildings, in which systems a passenger indicates his/her destination floor already in the elevator lobby before going into the elevator car. For giving destination calls the elevator lobbies are provided with destination operating panels. Destination operating panels are generally provided with a so-called decimal numeric keypad and a display means. If a passenger is going e.g. to floor 24, he/she keys into the decimal numeric keypad first the number 2 and then the number 4. The destination operating panel sends the data about the call-giving floor and about the aforementioned floor 24 to the control system of the elevator system. The control system comprised in the elevator system allocates the optimal elevator for the use of the passenger and transmits information about this to the call-giving panel, to the display means, on which appears e.g. the text “Elevator B”. In this way the elevator system identifies to the user in response to a destination call the elevator allocated to him/her. One problem is that the use of a decimal numeric keypad is slow. Another problem is that since a decimal numeric keypad enables, in principle, the keying in of any floor number whatsoever, this easily results in erroneous keyings. A passenger can, for example, key in a destination call to a floor that the elevator system does not serve or the floor is temporarily locked. For rectifying incorrect keying, the occurrence of an error must be indicated and the keying in must be performed again. This slows down use of the system. It is also possible that there is an access control system in use in the building, with which system the access of passengers to floors within the scope of the access control can be limited. Another problem in prior-art solutions is that a call-giving device can give to the user information relating to the destination floor poorly. For example, in the case of a decimal numeric keypad a call-giving device cannot give information to the user about the destination floor before the user has entered the destination floor into the device. Thus information about the destination floor cannot be efficiently offered to the user. Yet another problem is that solutions according to prior art do not enable rapid floor selection in a building having selectable floors with an identifier string that comprises three-digit floor numbers. For example, in the case of a decimal numeric keypad the production of a three-digit number is slow. Yet another problem of a solution utilizing a decimal numeric keypad is that it is restricted to very simple floor identifiers presented in numerical format. Yet another problem is that for a selection in solutions according to prior art a large number of icons, pushbuttons, et cetera, must be presented to a user. Thus, the display must be large in size and the solutions are not well suited to portable call-giving devices, in which the size of the display cannot be large. With regard to portable call-giving devices, a further problem is that the system does not necessarily know from which floor a call has come.